Ultrasonic imaging has found use in accurate measurement of structures of the eye, such as, for example, the cornea. Such measurements provide an ophthalmic surgeon valuable information that he can use to guide various surgical procedures performed on the cornea, one of the principal ones being the LASIK procedure for correcting refractive errors. They also provide diagnostic information after surgery has been performed to assess the geometrical location of corneal features such as the LASIK scar. This allows the surgeon to assess post surgical changes in the cornea as the cornea heals and to take steps to correct problems that can develop.
Ultrasonic imaging of the cornea presents a problem not generally encountered in other types of tissue. The corneal surfaces are necessarily smooth and spherically shaped to perform the optical function of focusing light rays. Because the corneal structures are smooth and regular, ultrasonic energy is reflected only in specific directions. In particular, an ultrasound beam from a transducer will only be reflected directly back to that transducer when the beam is aligned perpendicular to the corneal surface. This kind of reflective property is called specular reflection.
Because of the specular property of corneal surfaces, it will be appreciated that special care must be taken to align the transducer with the cornea at each position from which a partial image is to be formed. Ultrasonic imaging of large portions of the cornea can be accomplished by scanning the transducer along the cornea surface while continually adjusting the alignment of the transducer to provide a beam that is always directed toward the cornea's center of curvature.
Corneal imaging and measuring of corneal dimensions require that the scanning motion of the transducer be smooth and precisely aligned. Departures, even as small as 5 microns, of the transducer position from a circular path or of the beam's direction from the center of curvature can significantly degrade the resulting image. Mechanisms for performing the requisite scan alignment are described in U.S. Pat. Nos. 6,491,637 and 5,331,962 which are incorporated herein by reference. The reference “Ultrasonography of the Eye and Orbit”, Second Edition, Coleman et al., published by Lippincott Williams & Wilkins, 2006 contains an excellent historical and technical summary of ultrasonic imaging of the eye and is incorporated herein by this reference.
While ultrasonic imaging may be used by ophthalmologists for quantitative analysis of laser refractive surgery, it may also be used for implantation of corneal and phakic lenses, implantation of intraocular lenses and specialty procedures such as glaucoma and cataract treatment.
Except for on-axis measurements, dimensions of eye components behind the iris cannot be determined by optical means. New procedures such as implantation of accommodative lenses may provide nearly perfect vision without spectacles or contact lenses. Implantation of accommodative lenses requires precision measurements of, for example, the lens width for successful lens implantation. Ultrasonic imaging can be used to provide the required accurate images of the lens especially where it attaches to the ciliary muscle which is well off-axis and behind the iris and therefore not accessible to optical imaging.
It must be appreciated that ultrasonic imaging requires a liquid medium to be interposed between the object being imaged and the transducer, which requires in turn that the eye, the transducer, and the path between them be at all times be immersed in a liquid medium. Concern for safety of the cornea introduces the practical requirement that the liquid medium be either pure water or normal saline water solution. In either case, the entire mechanism or major portions of it must be submerged in water for long periods.
Conventional mechanical components for guiding and controlling the motion of the transducer, such as journal, ball or roller bearings, are ill-suited for underwater operation. Films inevitably form on the bearing components, interfering with their smooth operation. Anti-fouling solutions cannot be added to the water because they introduce an unacceptable risk of injury to the patient's eye even if the eye is separated from the main body of the liquid by a thin, ultrasonically transparent barrier. Leaks through the barrier film or accidental perforation of the barrier film are an ever present possibility in a practical clinical device.
There remains, therefore, a need for a versatile scan head and transducer positioning apparatus; a water-proof arc scanning motor; an accurate transducer locator method; a fluid bearing method that can provide smooth scanning motion; and a disposable eyepiece, all of which are necessary for an improved ultrasonic arc scanning apparatus that can provide precision imaging for ophthalmology and optometry applications.
Another challenge for any medical imaging system is to provide the highest possible image resolution while also attaining a high depth of image at a reasonable cost. Optical systems such as optical coherence tomography are compact and cost effective and provide excellent resolution. However, they are only capable of imaging a few millimeters into any opaque tissue surface as the light is rapidly absorbed. Current ultrasound systems are very compact and cost effective and have high tissue penetration depths of 100 mm or more. However, they offer relatively low resolution due to their low range of operating frequencies from about 5 MHz to about 10 MHz. MM systems are well-known imaging systems that provide both high depth of image and high resolution. However, they are characterized by high cost, large size and a costly dedicated infrastructure. High frequency ultrasound systems (from about 20 MHz to about 80 MHz) can provide high resolution but only with a limited image depth.
There remains a further need for a low cost, portable ultrasound imaging system that has substantially higher resolution than currently available devices and yet provides a depth of image of that is of high utility for medical diagnosticians.